The present invention relates to a process and a device for the production of spherical metal particles.
Such spherical metal particles are formed in particular as soldering beads, which are used for the electrical contacting of electronic components. One typical field of application in this connection is the contacting of integrated circuits on printed circuit boards. Soft soldering alloy beads in particular are used for this purpose, whose diameters are typically in the range between 0.1 and 1.5 mm. A decisive factor for a reproducible and defect-free contacting is that the soldering beads have as ideal a spherical shape as possible. In addition the variation in size of the soldering beads that are used should be as small as possible.
A process and a device for the production of spherical particles from a liquid phase are known from EP 0 467 221 B1. The particles to be produced consist in particular of metal or glass.
Droplets are formed from the liquid phase. For this purpose the liquid phase is fed to a vibrating nozzle.
The nozzle itself as well as a defined section connected to the outlet opening is maintained at a constant temperature that is 1° C. to 10° C. above the melting point of the liquid phase.
The droplets leaving the outlet opening still have a spherical shape. This spherical shape of the droplets is first of all formed during passage along the aforementioned section. The section is arranged vertically so that, due to the force of gravity, the droplets execute a falling movement along the section, during which the spherical shapes of the droplets are produced.
After passing through this section the spherical droplets are led into a gaseous or liquid coolant whose operating temperature is at least 100° C. below the melting point. The feed of the coolant preferably takes place in the same direction as the falling direction of the droplets. Due to the contact with the coolant the droplets are suddenly quenched, resulting in a consolidation of the droplets into the respective particles.
A significant disadvantage of this process is that when the droplets strike the surface of the coolant deformations of the said droplets occur, leading to variations of the particles from the desired spherical shape.
A further disadvantage is that the section between the outlet opening of the nozzle and surface of the coolant cannot or can only be incompletely screened against the ambient atmosphere. In the production of metal particles using oxidizable metal melts a partial, uncontrolled closure of the outlet opening of the nozzle then occurs. This leads in turn to undesirable variations in the size distributions of the metal particles.
DE 40 12 197 C2 describes a process for the production of spherical metal particles from a melt. The melt is introduced dropwise from a melting crucible into a stream of liquid that forms a coolant for cooling the droplets. An oil is preferably used as coolant. The liquid stream is in this connection led countercurrent to the falling direction of the droplets. A temperature gradient is formed within the liquid stream, with the result that the temperature of the liquid in the region of the outlet opening is at least as high as the melting point of the metal. The temperature of the liquid in the region of the liquid feed is thus so low that the droplets falling in the liquid stream solidify at least on their surfaces.
A disadvantage in this respect is that the liquid has to be heated to temperatures that correspond to the melting point of the metal, which leads to a marked thermal stress of the coolant. This in turn leads to an undesirably severe restriction on the availability of such systems, since the coolant has to be renewed within short time intervals. In addition there is the danger of the formation of cracked products in the region of the outlet opening of the nozzle. The flow of the liquid through the nozzle is thereby affected, resulting in undesirably high variations in the sizes of the metal spheres.